Furnace including perforated and bluff body flame holder for enhanced stability and turndown

ABSTRACT

A combustion system includes a perforated flame holder and a plurality of bluff body members positioned between the perforated flame holder and a fuel source. The fuel source outputs a fuel stream through gaps between the bluff body members toward the perforated flame holder. The perforated flame holder and the bluff body members collectively hold a combustion reaction supported by the fuel stream.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application which claims priority benefit under 35 U.S.C. § 120 (pre-AIA) of co-pending International Patent Application No. PCT/US2018/014227, entitled “FURNACE INCLUDING PERFORATED AND BLUFF BODY FLAME HOLDER FOR ENHANCED STABILITY AND TURNDOWN,” filed Jan. 18, 2018 (docket number 2651-304-04). International Patent Application No. PCT/US2018/014227 claims priority benefit from U.S. Provisional Patent Application No. 62/448,234, entitled “FURNACE INCLUDING PERFORATED AND BLUFF BODY FLAME HOLDER FOR ENHANCED STABILITY AND TURNDOWN,” filed Jan. 19, 2017 (docket number 2651-304-02). Each of the foregoing applications, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

SUMMARY

According to an embodiment, a combustion system includes a perforated flame holder positioned in a furnace volume, an oxidant source configured to introduce an oxidant into the furnace volume, and a fuel source configured to output a fuel stream including a fuel toward the perforated flame holder. The combustion system further includes an array of bluff body members positioned between the fuel source and the perforated flame holder. The bluff body members are separated from each other by gaps such that the fuel stream passes through the gaps toward the perforated flame holder. The array of bluff body members and the perforated flame holder are configured to stably support a combustion reaction of the fuel and oxidant. According to an embodiment, the perforated flame holder and the bluff body members are configured to hold the combustion reaction within the perforated flame holder and in a space between the perforated flame holder and the bluff body members.

According to an embodiment, a method includes supporting a perforated flame holder in a furnace volume and supporting an array of bluff body members in the furnace volume between the perforated flame holder and a fuel source. The method includes introducing an oxidant into the furnace volume and outputting a fuel stream including a fuel from the fuel source toward the perforated flame holder. The method includes passing the fuel stream through gaps between a plurality of bluff body members positioned between the fuel source and the perforated flame holder and supporting a combustion reaction of the fuel and oxidant with the perforated flame holder and the bluff body members. According to an embodiment, the method includes holding the combustion reaction within the perforated flame holder and in a space between the perforated flame holder and the bluff body members.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a combustion system, according to an embodiment.

FIG. 2 is a simplified diagram of a burner system, including a perforated flame holder configured to hold a combustion reaction, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flame holder of FIGS. 1 and 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner system including the perforated flame holder shown and described herein, according to an embodiment.

FIG. 5A is a simplified diagram of a burner system, including a perforated flame holder configured to hold a combustion reaction, according to an embodiment.

FIG. 5B is a side sectional diagram of a portion of the perforated flame holder of FIG. 5A, according to an embodiment.

FIG. 6A is a diagram of a combustion system including a perforated flame holder and an array of bluff body members, according to an embodiment.

FIG. 6B is a side sectional view of the perforated flame holder and the bluff body members of FIG. 6A, according to an embodiment.

FIG. 6C is a top view of the array of bluff body members of FIG. 6A, according to an embodiment.

FIG. 6D is an illustration of the combustion system of FIG. 6A in a standard operating state, according to an embodiment.

FIG. 7 is an illustration of a combustion system including a bracket configured to support a plurality of bluff body members, according to an embodiment.

FIG. 8A is an illustration of a combustion system including a perforated flame holder and an array of bluff body members, according to an embodiment.

FIG. 8B is a side view of the array of bluff body members of FIG. 9A, according to an embodiment.

FIG. 9A is an illustration of combustion system including a perforated flame holder and an array of bluff body members, according to an embodiment.

FIG. 9B is a side-sectional view of the array of bluff body members of FIG. 9A, according to an embodiment.

FIG. 10A is an illustration of combustion system including a perforated flame holder and an array of bluff body members, according to an embodiment. FIG. 10B is a top view of the bluff body members of the combustion system of FIG. 10A, according to an embodiment.

FIG. 11 is a flow diagram of a process for operating a combustion system, according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure.

FIG. 1 is a block diagram of a combustion system 100, according to an embodiment. The combustion system 100 includes a perforated flame holder 102, and a plurality of bluff body members 105 positioned in a furnace volume 101. The combustion system 100 also includes a fuel source 109 and an oxidant source 107. The bluff body members 105 are positioned between the fuel source 109 and the perforated flame holder 102.

According to an embodiment, the bluff body members 105 are arranged in an array. The bluff body members 105 are arranged such that there is a respective gap between each pair of adjacent bluff body members 105.

According to an embodiment, the fuel source 109 is configured to output the fuel stream including a fuel toward the perforated flame holder 102. Because the bluff body members 105 are positioned between the fuel source 109 and the perforated flame holder 102, the fuel stream passes through the array of bluff body members 105 as the fuel stream travels toward the perforated flame holder 102.

According to an embodiment, the oxidant source 107 is configured to output an oxidant into the furnace volume 101. The fuel stream entrains the oxidant as it travels toward the bluff body members 105, between the bluff body members 105, and toward the perforated flame holder 102.

According to an embodiment, the perforated flame holder 102 and the bluff body members 105 are configured to hold a stable combustion reaction of the fuel and oxidant. In particular, the perforated flame holder 102 and the bluff body members 105 are configured to hold the combustion reaction of the fuel and oxidant primarily within the perforated flame holder 102 and in a space between the bluff body members 105 and the perforated flame holder 102.

According to an embodiment, the bluff body members 105 help to stabilize a combustion reaction of the fuel and oxidant. The bluff body members 105 impart a bluff body effect on the fuel stream as the fuel stream travels toward the perforated flame holder 102. In particular, as the fuel stream impinges on the bluff body members 105, the fuel stream is forced to change trajectory as it flows around the bluff body members 105 and through the gaps between the bluff body members 105. This disturbance in the fuel stream helps to hold a flame front of the combustion reaction of the fuel and oxidant downstream from the bluff body members 105 nearest to the fuel stream. This stabilizes the combustion reaction and reduces the risk of flashback toward the fuel source 109.

According to an embodiment, the combination of the perforated flame holder 102 and the bluff body members 105 further stabilizes the combustion reaction of the fuel and oxidant. According to an embodiment, the combustion reaction is primarily confined to a volume between the bluff body members 105 nearest to the fuel source 109 and a surface of the perforated flame holder 102 distal to the fuel source 109. According to an embodiment, the presence of the perforated flame holder 102 compresses a distance over which the combustion reaction would occur in the absence of the perforated flame holder 102.

According to an embodiment, the perforated flame holder 102 is a ceramic perforated flame holder. According to an embodiment, the perforated flame holder 102 includes silicon carbide. According to an embodiment, the perforated flame holder 102 includes zirconia.

According to an embodiment, the perforated flame holder 102 includes a reticulated ceramic tile. According to an embodiment, the perforated flame holder 102 includes an input face, an output face, and a plurality of perforations extending between the input face and the output face. According to an embodiment, the perforated flame holder 102 includes a honeycomb tile having elongated apertures formed therethrough from the input face to the output face.

According to an embodiment, the bluff body members 105 include a ceramic material. According to an embodiment, the bluff body members 105 include silicon carbide. According to an embodiment, the bluff body members 105 include zirconia. According to an embodiment, the bluff body members 105 include alumina.

According to an embodiment, the bluff body members 105 include rods that extend in a direction transverse to a primary direction of the fuel stream. According to an embodiment, each rod is separated from each adjacent rod by a respective gap. The gap can be about as wide as the lateral dimension of the rods. Alternatively, the gap can be wider than a lateral dimension of the rods. Alternatively, the gap can be narrower than a lateral dimension of the rods.

According to an embodiment, the bluff body members 105 can include an elliptical cross section, a rounded oblong cross-section, an ovular cross-section, a circular cross section, a semicircular cross section, a square cross section, a rectangular cross section, a polygonal cross section, or another type of bluff body cross-section suitable for imparting a bluff body effect to the fuel stream as it passes between the bluff body members 105.

According to an embodiment, the combustion system 100 can include a preheating mechanism configured to preheat the perforated flame holder 102 to a threshold temperature at which the perforated flame holder 102 can sustain a combustion reaction of the fuel and oxidant within or adjacent to the perforated flame holder 102. In a preheating operating state of the combustion system 100, the preheating mechanism preheats the perforated flame holder 102 to the threshold temperature. After the preheating mechanism has heated the perforated flame holder 102 to the threshold temperature, the preheating mechanism is deactivated and the combustion system 100 enters a standard operating state in which the fuel source 109 outputs the fuel stream toward the perforated flame holder 102. According to an embodiment, the preheating mechanism can also preheat the bluff body members 105.

FIG. 2 is a simplified diagram of a burner system 200 including a perforated flame holder 102 configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous unless further definition is provided.

Experiments performed by the inventors have shown that perforated flame holders 102 described herein can support very clean combustion. Specifically, in experimental use of burner systems 200 ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O₂) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion.

According to embodiments, the burner system 200 includes a fuel and oxidant source 202 disposed to output fuel and oxidant into a furnace volume 204 to form a fuel and oxidant mixture 206. As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms furnace volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder 102 is disposed in the furnace volume 204 and positioned to receive the fuel and oxidant mixture 206.

FIG. 3 is a side sectional diagram 300 of a portion of the perforated flame holder 102 of FIGS. 1 and 2, according to an embodiment. Referring to FIGS. 2 and 3, the perforated flame holder 102 includes a perforated flame holder body 208 defining a plurality of perforations 210 aligned to receive the fuel and oxidant mixture 206 from the fuel and oxidant source 202. As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder 102, shall be considered synonymous unless further definition is provided. The perforations 210 are configured to collectively hold a combustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). In another application the fuel can include natural gas (mostly CH₄) or propane (C₃H₈). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the perforated flame holder body 208 can be bounded by an input face 212 disposed to receive the fuel and oxidant mixture 206, an output face 214 facing away from the fuel and oxidant source 202, and a peripheral surface 216 defining a lateral extent of the perforated flame holder 102. The plurality of perforations 210 which are defined by the perforated flame holder body 208 extend from the input face 212 to the output face 214. The plurality of perforations 210 can receive the fuel and oxidant mixture 206 at the input face 212. The fuel and oxidant mixture 206 can then combust in or near the plurality of perforations 210 and combustion products can exit the plurality of perforations 210 at or near the output face 214.

According to an embodiment, the perforated flame holder 102 is configured to hold a majority of the combustion reaction 302 within the perforations 210. For example, on a steady-state basis, more than half the molecules of fuel output into the furnace volume 204 by the fuel and oxidant source 202 may be converted to combustion products between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction 302 may be output between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction 302. As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations 210 can be configured to collectively hold at least 80% of the combustion reaction 302 between the input face 212 and the output face 214 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction 302 that was apparently wholly contained in the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, the perforated flame holder 102 can support combustion between the input face 212 and output face 214 when combustion is “time-averaged.” For example, during transients, such as before the perforated flame holder 102 is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face 214 of the perforated flame holder 102. Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face 212 of the perforated flame holder 102.

While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations 210, but the “glow” of combustion heat is dominated by a visible glow of the perforated flame holder 102 itself. In other instances, the inventors have noted transient “huffing” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face 212 of the perforated flame holder 102 and the fuel nozzle 218, within the dilution region D_(D). Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 210 of the perforated flame holder 102, between the input face 212 and the output face 214. In still other instances, the inventors have noted apparent combustion occurring downstream from the output face 214 of the perforated flame holder 102, but still a majority of combustion occurred within the perforated flame holder 102 as evidenced by continued visible glow from the perforated flame holder 102 that was observed.

The perforated flame holder 102 can be configured to receive heat from the combustion reaction 302 and output a portion of the received heat as thermal radiation 304 to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the furnace volume 204. As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body 208.

Referring especially to FIG. 3, the perforated flame holder 102 outputs another portion of the received heat to the fuel and oxidant mixture 206 received at the input face 212 of the perforated flame holder 102. The perforated flame holder body 208 may receive heat from the combustion reaction 302 at least in heat receiving regions 306 of perforation walls 308. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 306, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 308. In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face 212 to the output face 214 (i.e., somewhat nearer to the input face 212 than to the output face 214). The inventors contemplate that the heat receiving regions 306 may lie nearer to the output face 214 of the perforated flame holder 102 under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions 306 (or for that matter, the heat output regions 310, described below). For ease of understanding, the heat receiving regions 306 and the heat output regions 310 will be described as particular regions 306, 310.

The perforated flame holder body 208 can be characterized by a heat capacity. The perforated flame holder body 208 may hold thermal energy from the combustion reaction 302 in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy from the heat receiving regions 306 to heat output regions 310 of the perforation walls 308. Generally, the heat output regions 310 are nearer to the input face 212 than are the heat receiving regions 306. According to one interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via thermal radiation, depicted graphically as 304. According to another interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via heat conduction along heat conduction paths 312. The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions 306 to the heat output regions 310. In this way, the perforated flame holder 102 may act as a heat source to maintain the combustion reaction 302, even under conditions where a combustion reaction 302 would not be stable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes the combustion reaction 302 to begin within thermal boundary layers 314 formed adjacent to walls 308 of the perforations 210. Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder 102, it is apparent that at least a majority of the individual reactions occur within the perforated flame holder 102. As the relatively cool fuel and oxidant mixture 206 approaches the input face 212, the flow is split into portions that respectively travel through individual perforations 210. The hot perforated flame holder body 208 transfers heat to the fluid, notably within thermal boundary layers 314 that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture 206. After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction 302 occurs. Accordingly, the combustion reaction 302 is shown as occurring within the thermal boundary layers 314. As flow progresses, the thermal boundary layers 314 merge at a merger point 316. Ideally, the merger point 316 lies between the input face 212 and output face 214 that define the ends of the perforations 210. At some position along the length of a perforation 210, the combustion reaction 302 outputs more heat to the perforated flame holder body 208 than it receives from the perforated flame holder body 208. The heat is received at the heat receiving region 306, is held by the perforated flame holder body 208, and is transported to the heat output region 310 nearer to the input face 212, where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature.

In an embodiment, each of the perforations 210 is characterized by a length L defined as a reaction fluid propagation path length between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the term reaction fluid refers to matter that travels through a perforation 210. Near the input face 212, the reaction fluid includes the fuel and oxidant mixture 206 (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within the combustion reaction region 302, the reaction fluid may include plasma associated with the combustion reaction 302, molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face 214, the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant.

The plurality of perforations 210 can be each characterized by a transverse dimension D between opposing perforation walls 308. The inventors have found that stable combustion can be maintained in the perforated flame holder 102 if the length L of each perforation 210 is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers 314 to form adjacent to the perforation walls 308 in a reaction fluid flowing through the perforations 210 to converge at merger points 316 within the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion).

The perforated flame holder body 208 can be configured to convey heat between adjacent perforations 210. The heat conveyed between adjacent perforations 210 can be selected to cause heat output from the combustion reaction portion 302 in a first perforation 210 to supply heat to stabilize a combustion reaction portion 302 in an adjacent perforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 can further include a fuel nozzle 218, configured to output fuel, and an oxidant source 107 configured to output a fluid including the oxidant. For example, the fuel nozzle 218 can be configured to output pure fuel. The oxidant source 107 can be configured to output combustion air carrying oxygen, and optionally, flue gas.

The perforated flame holder 102 can be held by a perforated flame holder support structure 222 configured to hold the perforated flame holder 102 at a dilution distance D_(D) away from the fuel nozzle 218. The fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture 206 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through the dilution distance D_(D) between the fuel nozzle 218 and the perforated flame holder 102. Additionally or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D_(D). In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance D_(D) between the fuel nozzle 218 and the input face 212 of the perforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one or more fuel orifices 226 having an inside diameter dimension that is referred to as “nozzle diameter.” The perforated flame holder support structure 222 can support the perforated flame holder 102 to receive the fuel and oxidant mixture 206 at the distance D_(D) away from the fuel nozzle 218 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 206 at the distance D_(D) away from the fuel nozzle 218 between 100 times and 1100 times the nozzle diameter. Preferably, the perforated flame holder support structure 222 is configured to hold the perforated flame holder 102 at a distance about 200 times or more of the nozzle diameter away from the fuel nozzle 218. When the fuel and oxidant mixture 206 travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction 302 to produce minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.

The oxidant source 107, whether configured for entrainment in the furnace volume 204 or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source 202.

The support structure 222 can be configured to support the perforated flame holder 102 from a floor or wall (not shown) of the furnace volume 204, for example. In another embodiment, the support structure 222 supports the perforated flame holder 102 from the fuel and oxidant source 202. Alternatively, the support structure 222 can suspend the perforated flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The support structure 222 can support the perforated flame holder 102 in various orientations and directions.

The perforated flame holder 102 can include a single perforated flame holder body 208. In another embodiment, the perforated flame holder 102 can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 222 can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure 222 can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least twice a thickness dimension T between the input face 212 and the output face 214. In another embodiment, the perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least three times, at least six times, or at least nine times the thickness dimension T between the input face 212 and the output face 214 of the perforated flame holder 102.

In an embodiment, the perforated flame holder 102 can have a width dimension W less than a width of the furnace volume 204. This can allow the flue gas circulation path 224 from above to below the perforated flame holder 102 to lie between the peripheral surface 216 of the perforated flame holder 102 and the furnace volume wall (not shown).

Referring again to both FIGS. 2 and 3, the perforations 210 can be of various shapes. In an embodiment, the perforations 210 can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations 210 can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations 210 can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations 210 can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face 212 to the output face 214. In some embodiments, the perforations 210 can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations 210 may have lateral dimension D less then than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 210 has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations 210 has a lateral dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations 210 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as the total volume of all perforations 210 in a section of the perforated flame holder 102 divided by a total volume of the perforated flame holder 102 including body 208 and perforations 210. The perforated flame holder 102 should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder 102 can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder 102 can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx.

The perforated flame holder 102 can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder 102 can be formed to include mullite or cordierite. Additionally or alternatively, the perforated flame holder body 208 can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body 208 can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross sectional areas are also known.

The inventors have found that the perforated flame holder 102 can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C.

The perforations 210 can be parallel to one another and normal to the input and output faces 212, 214. In another embodiment, the perforations 210 can be parallel to one another and formed at an angle relative to the input and output faces 212, 214. In another embodiment, the perforations 210 can be non-parallel to one another. In another embodiment, the perforations 210 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 210 can be intersecting. The body 308 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from reticulated ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from a ceramic material that has been punched, bored or cast to create channels.

In another embodiment, the perforated flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 210 can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band.

The perforated flame holder body 208 can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body 208 can include discontinuous packing bodies such that the perforations 210 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systems including the perforated flame holder 102 provide such clean combustion.

According to an embodiment, the perforated flame holder 102 may act as a heat source to maintain a combustion reaction even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream 206 contacts the input face 212 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 206 is below a (conventional) lower combustion limit of the fuel component of the fuel stream 206—lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture 206 will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.).

The perforated flame holder 102 and systems including the perforated flame holder 102 described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O₂, i.e. an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O₂. Moreover, the inventors believe perforation walls 308 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx.

According to another interpretation, production of NOx can be reduced if the combustion reaction 302 occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder 102 is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder 102.

FIG. 4 is a flow chart showing a method 400 for operating a burner system including the perforated flame holder shown and described herein. To operate a burner system including a perforated flame holder, the perforated flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 400 begins with step 402, wherein the perforated flame holder is preheated to a start-up temperature, T_(S). After the perforated flame holder is raised to the start-up temperature, the method proceeds to step 404, wherein the fuel and oxidant are provided to the perforated flame holder and combustion is held by the perforated flame holder.

According to a more detailed description, step 402 begins with step 406, wherein start-up energy is provided at the perforated flame holder. Simultaneously or following providing start-up energy, a decision step 408 determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, T_(S). As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps 406 and 408 within the preheat step 402. In step 408, if the temperature T of at least a predetermined portion of the perforated flame holder is greater than or equal to the start-up temperature, the method 400 proceeds to overall step 404, wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder.

Step 404 may be broken down into several discrete steps, at least some of which may occur simultaneously.

Proceeding from step 408, a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step 410. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the perforated flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the perforated flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder.

Proceeding to step 412, the combustion reaction is held by the perforated flame holder.

In step 414, heat may be output from the perforated flame holder. The heat output from the perforated flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step 416, a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the perforated flame holder.

Proceeding to decision step 418, if combustion is sensed not to be stable, the method 400 may exit to step 424, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step 402, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in step 418, combustion in the perforated flame holder is determined to be stable, the method 400 proceeds to decision step 420, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 404) back to step 410, and the combustion process continues. If a change in combustion parameters is indicated, the method 400 proceeds to step 422, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 404) back to step 410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step 422. Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the perforated flame holder over one or more iterations of the loop within step 404.

Referring again to FIG. 2, the burner system 200 includes a heater 228 operatively coupled to the perforated flame holder 102. As described in conjunction with FIGS. 3 and 4, the perforated flame holder 102 operates by outputting heat to the incoming fuel and oxidant mixture 206. After combustion is established, this heat is provided by the combustion reaction 302; but before combustion is established, the heat is provided by the heater 228.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 228 can include a flame holder configured to support a flame disposed to heat the perforated flame holder 102. The fuel and oxidant source 202 can include a fuel nozzle 218 configured to emit a fuel stream 206 and an oxidant source 107 configured to output oxidant (e.g., combustion air) adjacent to the fuel stream 206. The fuel nozzle 218 and oxidant source 107 can be configured to output the fuel stream 206 to be progressively diluted by the oxidant (e.g., combustion air). The perforated flame holder 102 can be disposed to receive a diluted fuel and oxidant mixture 206 that supports a combustion reaction 302 that is stabilized by the perforated flame holder 102 when the perforated flame holder 102 is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated perforated flame holder 102.

The burner system 200 can further include a controller 230 operatively coupled to the heater 228 and to a data interface 232. For example, the controller 230 can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the perforated flame holder 102 needs to be pre-heated and to not hold the start-up flame when the perforated flame holder 102 is at an operating temperature (e.g., when T≥T_(S)).

Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture 206 to cause heat-recycling and/or stabilizing vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 206 to cause the fuel and oxidant mixture 206 to proceed to the perforated flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a perforated flame holder 102 operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater 228 may include an electrical power supply operatively coupled to the controller 230 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 206. An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture 206. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 228 may include an electrical resistance heater configured to output heat to the perforated flame holder 102 and/or to the fuel and oxidant mixture 206. The electrical resistance heater can be configured to heat up the perforated flame holder 102 to an operating temperature. The heater 228 can further include a power supply and a switch operable, under control of the controller 230, to selectively couple the power supply to the electrical resistance heater.

An electrical resistance heater 228 can be formed in various ways. For example, the electrical resistance heater 228 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the perforations 210 defined by the perforated flame holder body 208. Alternatively, the heater 228 can include an inductive heater, a high-energy beam heater (e.g. microwave or laser), a frictional heater, electro-resistive ceramic coatings, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 228 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the oxidant and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture 206 that would otherwise enter the perforated flame holder 102. The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 230, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture 206 in or upstream from the perforated flame holder 102 before the perforated flame holder 102 is heated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operatively coupled to the control circuit 230. The sensor 234 can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder 102. The control circuit 230 can be configured to control the heating apparatus 228 responsive to input from the sensor 234. Optionally, a fuel control valve 236 can be operatively coupled to the controller 230 and configured to control a flow of fuel to the fuel and oxidant source 202. Additionally or alternatively, an oxidant blower or damper 238 can be operatively coupled to the controller 230 and configured to control flow of the oxidant (or combustion air).

The sensor 234 can further include a combustion sensor operatively coupled to the control circuit 230, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder 102. The fuel control valve 236 can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source 202. The controller 230 can be configured to control the fuel control valve 236 responsive to input from the combustion sensor 234. The controller 230 can be configured to control the fuel control valve 236 and/or oxidant blower or damper to control a preheat flame type of heater 228 to heat the perforated flame holder 102 to an operating temperature. The controller 230 can similarly control the fuel control valve 236 and/or the oxidant blower or damper to change the fuel and oxidant mixture 206 flow responsive to a heat demand change received as data via the data interface 232.

FIG. 5A is a simplified perspective view of a combustion system of 500, including another alternative perforated flame holder 102, according to an embodiment. The perforated flame holder 102 of FIG. 5A is a reticulated ceramic perforated flame holder including a discontinuous perforated flame holder body 208 with branching perforations, according to an embodiment. FIG. 5B is a simplified side sectional diagram of a portion of the perforated flame holder 102 of FIG. 5B, according to an embodiment. The reticulated ceramic perforated flame holder 102 of FIGS. 5A, 5B can be implemented in the various combustion systems described herein, according to an embodiment.

Referring to FIGS. 5A and 5B, the perforated flame holder body 208 can be discontinuous. The perforated flame holder body 208 can define perforations 210 that branch from one another. The perforated flame holder body 208 can include stacked sheets of material, each sheet having openings non-registered to the openings of a subjacent or superjacent sheet. “Non-registered” openings (described below) refer to openings that cause branching of oxidation fluid flow paths. “Non-registered” openings may, in fact, correspond to patterns that have preplanned differences in location from one another. “Registered” openings, which cause the perforations 210 to be separated from one another, may also have preplanned differences in location from one sheet to another (or may be superpositioned to one another) but “registered” openings do not cause branching, and hence the perforations 210 are separated from one another.

According to an embodiment, the perforated flame holder body 208 can include fibers 539 including reticulated fibers. The fibers 539 can define branching perforations 210 that weave around and through the fibers 539.

The fibers 539 can include an alumina silicate. For example, the fibers 539 can be formed from mullite or cordierite. In another embodiment, the fibers 539 can be formed from silicon carbide or zirconia. In another embodiment, the fibers 539 can include a metal. For example, the fibers 539 can include stainless steel and/or a metal superalloy.

The term “reticulated fibers” refers to a netlike structure. In one embodiment, the fibers 539 are formed from a ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant, the combustion reaction, and heat transfer to and from the perforated flame holder body 208 functions similarly to the embodiment shown and described above with respect to FIGS. 2-4. One difference in activity is a mixing between perforations 210, because the fibers 539 form a discontinuous perforated flame holder body 208 that allows flow back and forth between neighboring perforations.

According to an embodiment, the network of reticulated fibers 539 is sufficiently open for downstream fibers 539 to emit radiation for receipt by upstream fibers 539 for the purpose of heating the upstream fibers 539 sufficiently to maintain combustion of a lean fuel and oxidant mixture. Compared to a continuous perforated flame holder body 208, heat conduction paths 312 between fibers 539 are reduced owing to separation of the fibers 539. This may cause relatively more heat to be transferred from the heat receiving region (heat receiving area) to the heat output region (heat output area) of the perforation wall via thermal radiation.

Various dimensions and relationships between pores and reticulated fibers have been found to be effective. According to embodiments, the reticulated ceramic body may have between 3 and 30 pores per inch. This measurement is typically along any chosen line across a surface of the perforated flame holder 102.

FIG. 6A is a diagram of a combustion system 600, according to an embodiment. The combustion system 600 includes an array of bluff body members 605 and a perforated flame holder 102 positioned in a furnace volume 601. The combustion system 600 includes a support structure 640 that supports the bluff body members 605 and the perforated flame holder 102 within the furnace volume 601. The combustion system 600 also includes a fuel nozzle 618 and an oxidant source 107.

According to an embodiment, the fuel source 109 is configured to output a fuel stream including a fuel toward the perforated flame holder 102. Because the bluff body members 605 are positioned between the fuel nozzle 618 and the perforated flame holder 102, the fuel stream passes through the array of bluff body members 605 as it travels toward the perforated flame holder 102.

According to an embodiment, the oxidant source 107 is configured to output an oxidant into the furnace volume 601. The fuel stream entrains the oxidant as it travels toward the perforated flame holder 102.

According to an embodiment, the bluff body members 605 are arranged in an array of three rows of bluff body members 605. Each row of bluff body members 605 includes a plurality of bluff body members 605 spaced horizontally from each other and separated by a lateral separation distance D_(H) (see FIG. 6B). The rows of bluff body members 605 are separated from each other vertically by a separation distance D_(V). Each bluff body member 605 can have a thickness T.

In an embodiment, each bluff body member can have a lateral dimension of between 0.25 and 2.5 inches. In one embodiment, the thickness T is about 0.25 inches. In one embodiment, the lateral separation distance DH is about 0.5 inches. In one embodiment, the vertical separation distance DV is about 0.5 inches.

According to an embodiment, the lateral separation distance D_(H) can be approximately the same as the thickness T of one of the bluff body members 605. According to an embodiment, the lateral separation distance D_(H) can be greater than a thickness T of one of the bluff body members 605. According to an embodiment, the lateral separation distance D_(H) can be less than a thickness T of the bluff body members 605. According to an embodiment, the separation distance D_(H) can be between 0.25 inch and 2 inches.

According to an embodiment, the vertical separation distance D_(V) can be approximately the same as a thickness T of one of the bluff body members 605. According to an embodiment, the vertical separation distance D_(V) can be greater than a thickness T of one of the bluff body members 605. According to an embodiment, the vertical separation distance D_(V) can be less than a thickness of the bluff body members 605. According to an embodiment, the separation distance D_(V) can be between 0.25 inch and 2 inches.

According to an embodiment, the bluff body members 605 can include rods. According to an embodiment, the rods can have a circular cross-section. According to an embodiment, the rods can have an elliptical cross-section. According to an embodiment, the rods can have an ovular cross-section. According to an embodiment, the rods can have a semicircular cross-section. According to an embodiment, the rods can have a square cross-section (or square with eased or quarter round edges). According to an embodiment, the rods can have a rectangular cross-section. According to an embodiment, the rods can have a polygonal cross-section. According to an embodiment, the rods can have an I-beam cross-section. According to an embodiment, the rods can have a C-channel shaped cross-section. According to an embodiment, the rods can have an L-type or V-type cross-section. According to an embodiment, the rods can have a bluff body cross-section other than those listed above.

According to an embodiment, the bluff body members 605 can be between 0.25 inch and 1 inches in thickness.

In one embodiment, the bluff body members 605 extend in a direction transverse to a primary direction of a fuel stream output by the fuel nozzle 618. The cylindrical rods can be spaced apart.

In one embodiment, the bluff body members 605 include a ceramic material. The ceramic material can include silicon carbide, zirconia, alumina, or other ceramic material suitable for high temperature environments.

In one embodiment, the support structure 640 includes support legs 642. The perforated flame holder 102 is positioned on top of the support legs 642. Alternatively, the perforated flame holder 102 can be supported by the support legs 642 in another suitable manner. In one embodiment, the bluff body members 605 are supported by the support legs 642. In particular, the bluff body members 605 can be supported by brackets extending between the support legs 642. According to an embodiment, the bluff body members 605 can fit within grooves, slots, or apertures within brackets or support members extending between the support legs 642.

According to an embodiment, the support structure 640 can include refractory bricks 644. The refractory bricks 644 can be positioned on a floor 646 of the furnace. The support legs 642 can be mounted on or supported by the refractory bricks 644. Alternatively, the refractory bricks 644 may not be present and the support legs 642 can extend all the way to the floor 646 or to another support structure 640 coupled to the floor 646.

According to an embodiment, the fuel nozzle 618 extends through an aperture 648 in the floor 646 of the furnace. Alternatively, the fuel nozzle 618 can be positioned entirely below the aperture 648 in the floor 646 of the furnace. The fuel nozzle 618 can output a fuel stream including a fuel toward the bluff body members 605 and the perforated flame holder 102.

According to an embodiment, the oxidant source 107 is configured to output an oxidant into the furnace volume 601 through the aperture 648 in the floor 646. The oxidant source 107 can include a blower, a forced draft system, a barrel register, or another suitable system or device for providing an oxidant into the furnace volume 601. The oxidant source 107 can provide the oxidant into the furnace volume 601 other than through the aperture 648 in the floor 646.

According to an embodiment, the fuel nozzle 618 is configured to output the fuel stream such that the fuel stream impinges upon the bluff body members 605 and passes through the gaps between the bluff body members 605. A portion of the fuel stream passes around and between the bluff body members 605 and impinges on the perforated flame holder 102.

According to an embodiment, in some cases as the fuel stream impinges on and passes around the bluff body members 605, the bluff body members 605 can cause a vortex wake in the fuel stream downstream from the bluff body members 605. The vortex wake can enhance mixing of the fuel and oxidant such that the fuel stream entrains the oxidant more thoroughly as the fuel stream continues toward the perforated flame holder 102.

According to an embodiment, in some cases as the fuel stream impinges on and passes around the bluff body members 605, the bluff body members 605 can cause a turbulent boundary layer in the fuel stream downstream from the bluff body members 605. The turbulent boundary layer can also enhance mixing of the fuel and oxidant such that the fuel stream entrains the oxidant more thoroughly as the fuel stream continues toward the perforated flame holder 102.

According to an embodiment, the perforated flame holder 102 and the bluff body members 605 hold a combustion reaction of the fuel and oxidant primarily within the perforated flame holder 102 and within a volume between the bluff body members 605 and the perforated flame holder 102.

According to an embodiment, the combination of the perforated flame holder 102 and the bluff body members 605 stabilizes the combustion reaction of the fuel and oxidant in comparison to a situation in which either bluff body members 605 or the perforated flame holder 102 are not present. According to an embodiment, the combustion reaction is primarily confined to a volume between the bluff body members 605 nearest to the fuel nozzle 618 and a surface of the perforated flame holder 102 distal to the fuel nozzle 618. According to an embodiment, the presence of the perforated flame holder 102 confines the combustion reaction over a smaller distance than if the perforated flame holder 102 was not present.

According to an embodiment, the combustion system 600 can include a preheating mechanism configured to preheat the perforated flame holder 102 to a threshold temperature at which the perforated flame holder 102 can sustain a combustion reaction of the fuel and oxidant within or adjacent to the perforated flame holder 102. In a preheating operating state of the combustion system 600, the preheating mechanism preheats the perforated flame holder 102 to the threshold temperature. After the preheating mechanism has heated the perforated flame holder 102 to the threshold temperature, the preheating mechanism is deactivated and the combustion system 600 enters a standard operating state in which the fuel source 109 outputs the fuel stream toward the perforated flame holder 102.

According to an embodiment, the preheating mechanism can also preheat the bluff body members 605 to a threshold temperature at which the bluff body members 605 can support a combustion reaction of the fuel and oxidant.

According to an embodiment, the first row of bluff body members 605 is positioned between 6 inches and 3 feet above the fuel nozzle 618. According to an embodiment, the first row of bluff body members 605 is positioned about 12 inches above the fuel nozzle 618.

FIG. 6B is a side sectional view of the perforated flame holder 102 and the bluff body members 605. As seen in FIG. 6B, the array of bluff body members 605 includes three rows of bluff body members 605. In the example of FIG. 6B, each row of bluff body members 605 includes seven bluff body members 605. The bluff body members 605 in each row are spaced apart from each other by gaps of width D_(H) through which the fuel stream can pass as it travels toward the perforated flame holder 102. The rows of bluff body members 605 are spaced apart from each other by the vertical separation distance D_(V). As the fuel stream passes between and around the bluff body members 605 toward the perforated flame holder 102, the bluff body members 605 affect the fuel stream causing enhanced mixing of the fuel stream and the oxidant. The bluff body members 605 also reduce the risk of flashback of the combustion reaction toward the fuel nozzle 618.

According to an embodiment, the rows of bluff body members 605 can be offset laterally with respect to each other such that the bluff body members 605 of the middle row may be positioned directly above a gap between the bluff body members 605 of the row closest to the fuel nozzle 618. According to an embodiment, the gaps D_(H) between adjacent pairs of bluff body members 605 can vary such that not all gaps are equal.

According to an embodiment, there can be more or fewer numbers of rows of bluff body members 605 than are shown in FIG. 6B. For example, there may be only a single row of bluff body members 605. Alternatively, there can be two or four or more rows of bluff body members 605.

In FIG. 6B, the bluff body members 605 have circular cross sections and have a thickness (or diameter) T. According to an embodiment, the bluff body members 605 can have a cross-section that is not circular, such as an elliptical or ovular cross-section.

FIG. 6C is a top view of the array of bluff body members 605, according to an embodiment. Only the top row of bluff body members 605 is visible in the view of FIG. 6C. The bluff body members 605 extend in a direction transverse to the primary direction of the fuel stream.

FIG. 6D is an illustration of the combustion system 600 in a standard operating state in which the fuel nozzle 618 is emitting a fuel stream 650. The perforated flame holder 102 and the array of bluff body members 605 collectively hold a combustion reaction 652 of the fuel and oxidant primarily within the perforated flame holder 102 and between the bluff body members 605 and the perforated flame holder 102.

FIG. 7 is an illustration of a combustion system 700 including a bracket 754 configured to support the bluff body members 605. The bracket 754 is coupled to the support legs 642 of the support structure 640. The bracket 754 includes apertures 756 through which support members 605 can extend. The ends of the support members 605 extend through the apertures 756 and rest on the bracket 754. The bracket 754 includes three rows of apertures 756 such that three rows of the bluff body members 605 can be supported in the bracket 754. However, for simplicity, only a single row of bluff body members 605 is illustrated in FIG. 7. Furthermore, the perforated flame holder 102 is not pictured in FIG. 7 in order to more clearly illustrate the positioning of the bluff body members 605 in the bracket 754.

According to an embodiment, a second bracket 754 (not pictured in FIG. 7) supports the opposite ends of the bluff body members 605. The second bracket 754 is coupled to two other support legs 642 of the support structure 640.

According to an embodiment, the bracket 754 can include a ceramic material such as zirconia, silicon carbide, alumina, or other ceramic materials suitable for high temperature environments.

According to an embodiment, the support structure 640 can include other devices and structures for supporting the of bluff body members 605 relative to the perforated flame holder 102 and the fuel nozzle 618. Those of skill in the art will recognize, in light of the present disclosure, that many other kinds of structures can be used to support the bluff body members 605. All such other structures fall within the scope of the present disclosure.

FIG. 8A is an illustration of a combustion system 800 including the perforated flame holder 102, the array of bluff body members 605, a support structure 640. The support structure 640 is configured to support the bluff body members 605 and perforated flame holder 102 in a furnace volume 801. The combustion system 800 includes a fuel nozzle 618 configured to output a fuel stream 650. The combustion system 800 can be substantially similar to the combustion system 600 of FIG. 6A, except that only two rows of bluff body members 605 are present in the combustion system 800. The perforated flame holder 102 and the bluff body members 605 collectively hold a combustion reaction 652 of the fuel and oxidant.

FIG. 8B is a side view of the array of bluff body members 605 of FIG. 8A. As shown in FIG. 8B, the first and second rows of bluff body members 605 are laterally offset relative to one another such that the bluff body members 605 of the top row are positioned directly above gaps between the bluff body members 605 of the bottom row. Alternatively, the bluff body members 605 of the top row can be positioned directly over the bluff body members 605 of the bottom row.

FIG. 9A is an illustration of combustion system 900 including the perforated flame holder 102, the array of bluff body members 605, a support structure 640 configured to support the bluff body members 605 and perforated flame holder 102, and a fuel nozzle 618 configured to output a fuel stream 650. The combustion system 900 can be substantially similar to the combustion system 600 of FIG. 6A, except that only a single row of bluff body members 605 is present in the combustion system 900. The perforated flame holder 102 and the bluff body members 605 collectively hold a combustion reaction 652 of the fuel and oxidant.

FIG. 9B is a side view of the array of bluff body members 605 of FIG. 9A.

FIG. 10A is an illustration of combustion system 1000 including the perforated flame holder 102, the array of bluff body members 605, a support structure 640 configured to support the bluff body members 605 and perforated flame holder 102 in a furnace volume 1001, and a fuel nozzle 618 configured to output a fuel stream 650. The combustion system 1000 can be substantially similar to the combustion system 600 of FIG. 6A, except that the middle row of bluff body members 605 extends in a direction transverse or perpendicular to the bluff body members 605 of the top and bottom rows. The perforated flame holder 102 and the bluff body members 605 collectively hold a combustion reaction 652 of the fuel and oxidant.

FIG. 10B is a top view of the bluff body members 605 of the combustion system 1000 of FIG. 10A. The top row of bluff body members 605 extends in a direction perpendicular to the middle row of bluff body members 605. Thus, the bluff body members 605 of the combustion system 1000 are arranged in a crisscrossing configuration.

FIG. 11 is a flow diagram of a process 1100 for operating a combustion system, according to an embodiment. At 1102 a perforated flame holder is supported in a furnace volume, according to an embodiment. At 1104 a plurality of bluff body members are supported in the furnace volume between the perforated flame holder and a fuel source, according to an embodiment. At 1106 an oxidant is introduced into the furnace volume, according to an embodiment. At 1108 a fuel stream including a fuel is passed from the fuel source through gaps between the bluff body members towards the perforated flame holder, according to an embodiment. At 1110 a combustion reaction of the fuel and oxidant is supported within the perforated flame holder, according to an embodiment.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

What is claimed is:
 1. A combustion system, comprising: a perforated flame holder positioned in a furnace volume; an oxidant source configured to introduce an oxidant into the furnace volume; a fuel source configured to output a fuel stream including a fuel toward the perforated flame holder; and a plurality of bluff body members positioned between the fuel source and the perforated flame holder, the bluff body members being separated from each other by gaps selected to allow the fuel stream to pass through the gaps toward the perforated flame holder, the plurality of bluff body members and the perforated flame holder being configured to collectively support a combustion reaction of the fuel and the oxidant within the perforated flame holder.
 2. The combustion system of claim 1, wherein the bluff body members and the perforated flame holder are configured to support the combustion reaction within the perforated flame holder and in a space between the bluff body members and the perforated flame holder.
 3. The combustion system of claim 1, further comprising a support structure holding the bluff body members.
 4. The combustion system of claim 3, wherein the support structure is configured to support the perforated flame holder.
 5. The combustion system of claim 4, wherein the plurality of bluff body members includes: a first row of bluff body members; and a second row of bluff body members positioned between the first row of bluff body members and the perforated flame holder.
 6. The combustion system of claim 5, wherein the bluff body members of the first row extend in a first direction and the bluff body members of the second row extend in a second direction transverse to the first direction.
 7. The combustion system of claim 5, wherein the bluff body members of the first row extend parallel to the bluff body members of the second row.
 8. The combustion system of claim 7, wherein the bluff body members of the second row are aligned with the gaps between the bluff body members of the first row.
 9. The combustion system of claim 7, wherein the bluff body members of the second row are aligned with the bluff body members of the first row.
 10. The combustion system of claim 5, wherein the plurality of bluff body members includes a third row of bluff body members positioned between the second row of bluff body members and the perforated flame holder.
 11. The combustion system of claim 3, wherein the support structure includes: a plurality of support legs supporting the perforated flame holder and the bluff body members above the fuel source; and a bracket coupled to one or more of the support legs and configured to hold the bluff body members.
 12. The combustion system of claim 11, wherein the bracket includes a plurality of apertures configured to support end portions of the bluff body members.
 13. The combustion system of claim 11, wherein the support structure includes a ceramic material.
 14. The combustion system of claim 13, wherein the support structure is entirely ceramic.
 15. The combustion system of claim 1, wherein the bluff body members have a circular cross-section.
 16. The combustion system of claim 1, wherein the bluff body members have an elliptical cross-section.
 17. The combustion system of claim 1, wherein the bluff body members have an ovular cross-section.
 18. The combustion system of claim 1, wherein characteristics of the fuel stream and the bluff body members are configured to introduce a vortex motion into the fuel stream downstream from the bluff body members.
 19. The combustion system of claim 1, wherein characteristics of the fuel stream and the bluff body members are configured to introduce a turbulent wake downstream from the bluff body members.
 20. The combustion system of claim 1, wherein the perforated flame holder is a reticulated ceramic perforated flame holder.
 21. The combustion system of claim 20, wherein the perforated flame holder includes a plurality of reticulated fibers.
 22. The combustion system of claim 21, wherein the perforated flame holder includes zirconia.
 23. The combustion system of claim 21, wherein the perforated flame holder includes alumina silicate.
 24. The combustion system of claim 21, wherein the perforated flame holder includes silicon carbide.
 25. The combustion system of claim 21, wherein the reticulated fibers are formed from extruded mullite.
 26. The combustion system of claim 21, wherein the reticulated fibers are formed from cordierite.
 27. The combustion system of claim 21, wherein the perforated flame holder is configured to support a combustion reaction of the fuel and the oxidant upstream, downstream, and within the perforated flame holder.
 28. The combustion system of claim 21, wherein the perforated flame holder has 3 to 30 pores per linear inch of surface area.
 29. The combustion system of claim 21, the perforated flame holder includes a plurality of perforations formed as passages between the reticulated fibers.
 30. The combustion system of claim 29, wherein the perforations are branching perforations.
 31. The combustion system of claim 29, wherein the perforated flame holder includes: an input face proximal to the fuel source; and an output face distal to the fuel source.
 32. The combustion system of claim 31, wherein the perforations extend between the input face and the output face.
 33. The combustion system of claim 31, wherein the input face corresponds to an extent of the reticulated fibers proximal to the fuel source.
 34. The combustion system of claim 33, wherein the output face corresponds to an extent of the reticulated fibers distal to the fuel source.
 35. The combustion system of claim 31, wherein the perforated flame holder is configured to support at least a portion of the combustion reaction within the perforated flame holder between the input face and the output face.
 36. The combustion system of claim 1, wherein one or more of the gaps is between 0.25 inch and 3 inches.
 37. The combustion system of claim 36, wherein one or more of the gaps is about equal to a thickness of one of the bluff body members.
 38. The combustion system of claim 37, wherein the thickness of the one of the bluff body members is about 0.25 inches.
 39. The combustion system of claim 1, wherein the fuel source includes a fuel nozzle.
 40. A method, comprising: supporting a perforated flame holder in a furnace volume; supporting a plurality of bluff body members in the furnace volume between the perforated flame holder and a fuel source; introducing an oxidant into the furnace volume; passing a fuel stream including a fuel from the fuel source through gaps between the bluff body members towards the perforated flame holder; and supporting a combustion reaction of the fuel and the oxidant within the perforated flame holder.
 41. The method of claim 40, further comprising supporting the combustion reaction of the fuel and the oxidant in a space between the perforated flame holder and the bluff body members.
 42. The method of claim 40, further comprising introducing a vortex motion into the fuel stream with the bluff body members.
 43. The method of claim 40, further comprising introducing a turbulent motion into the fuel stream with the bluff body members.
 44. The method of claim 43, further comprising entraining the oxidant in the fuel stream.
 45. The method of claim 44, further comprising enhancing entrainment of the oxidant by introducing a vortex motion into the fuel stream with the bluff body members.
 46. The method of claim 45, further comprising enhancing entrainment of the oxidant by introducing a turbulent wake in the fuel stream with the bluff body members.
 47. The method of claim 40, wherein the perforated flame holder is a reticulated ceramic perforated flame holder.
 48. The method of claim 47, wherein the perforated flame holder includes a plurality of reticulated fibers.
 49. A combustion system, comprising: a perforated flame holder positioned in a furnace volume; an oxidant source configured to introduce an oxidant into the furnace volume; a fuel source configured to output a fuel stream including a fuel toward the perforated flame holder; and a plurality of bluff body members positioned between the fuel source and the perforated flame holder and configured to perturb the fuel stream as the fuel stream passes between the bluff body members towards the perforated flame holder, the perforated flame holder being configured to collectively support a combustion reaction of the fuel and the oxidant within the perforated flame holder.
 50. The combustion system of claim 49, further comprising a support structure supporting the bluff body members within the furnace volume.
 51. The combustion system of claim 50, wherein the support structure supports the perforated flame holder within the furnace volume.
 52. The combustion system of claim 49, wherein the bluff body members and the perforated flame holder are configured to support the combustion reaction within the perforated flame holder and in a space between the bluff body members and the perforated flame holder.
 53. The combustion system of claim 49, wherein characteristics of the fuel stream and the bluff body members are configured to perturb the fuel stream by introducing a vortex motion into the fuel stream downstream from the bluff body members.
 54. The combustion system of claim 49, wherein characteristics of the fuel stream and the bluff body members are configured to perturb the fuel stream by introducing a turbulent wake downstream from the bluff body members. 